A conventional rectifying circuit which rectifies an AC power source to convert it into a DC power source has been disclosed in FIGS. 4 and 6 of Japanese Patent Laid-Open No. 59-198873 published on Nov. 10, 1984 in the title of "Power source rectifying circuit" which is provided with a circuit for suppressing higher harmonics in the source current. According to this publication, a switching element is connected to an output terminal of the rectifier circuit, a current waveform is compared with a synchronizing error signal obtained by multiplying a difference between the DC output voltage and a setpoint voltage by a voltage signal of the AC power source, and the switching element is turned on and off depending upon the polarity of the difference.
According to the above-mentioned prior art, the AC current instruction waveform for improving the power factor is formed relying upon an AC voltage waveform. When the AC voltage undergoes a change or when noise is generated the current instruction waveform is directly affected imposing a problem with regard to reliability.
Furthermore, a device must be provided to detect AC voltage waveforms, thus causing the circuit to become complex and bulky.